What did William Herschel discover?

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William Herschel is credited with discovering the planet Uranus in 1781. Herschel was using a telescope to search for comets when he came across the planet in the constellation Gemini. Once he recognized the object as a planet, he named it Georgium Sidus in honor of England's King George III.

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Astronomers outside of England rejected that name and eventually settled on the name Uranus, father of Saturn (or Cronus) in Greek mythology. In deference to Herschel, the satellites of Uranus were named after literary characters in Shakespeare and Alexander Pope.

Nearly a century earlier, the astronomer Flamsteed had observed the planet in the same area of the sky, but, mistaking it for a star, designated it as 34 Tauri in his catalog. When astronomers calculated the orbit of Uranus, they discovered the planet took 84 years to circle the sun one time. This is almost three times as long as the orbital period of Saturn, the farthest naked-eye planet from Earth. Using Kepler's third law, which states the square of a planet's orbital period is approximately equal to the cube of its distance from the sun in astronomical units (the average distance from Earth to the sun), they discovered that Uranus is located over 19 times farther from the sun than the Earth's orbit. This works out to an incredible distance of 1.78 billion miles away.

Using Earth-based telescopes, astronomers found that Uranus has five large satellites as well as a set of rings much dimmer than those of Saturn. In 1986, the Voyager 2 spacecraft did a flyby of Uranus, discovering several small moons and taking many photographs of the planet's nearly featureless atmosphere.